1. Field of the Invention
The present invention relates to a semiconductor fabrication process. More particularly, the present invention relates to a method for forming a contact opening.
2. Description of the Related Art
FIGS. 1A through 1C are schematic cross-sectional views showing the steps in the conventional method for fabricating a contact opening in a thin film transistor (TFT). As shown in FIG. 1A, the thin film transistor 100 mainly comprises a gate 102, a gate insulation layer 104, a channel layer 106 and a source/drain 108. Furthermore, a protective layer 110 covers the source/drain 108 to prevent any damage to the source/drain 108 in subsequent processes. Through a contact opening (not shown in FIG. 1A), the source/drain 108 is electrically connected to other conductive layers for receiving signals from external circuits.
In the conventional method of fabricating the contact opening, a patterned photoresist layer 112 is formed on the protection layer 110. Then, as shown in FIG. 1B, the high oxygen-content plasma is used as the reactive ions (labeled 113 in FIG. 1B) to perform a reactive ion etching (RIE) operation to remove a portion of the protective layer 110 using the patterned photoresist layer 112 as a mask. Finally, as shown in FIG. 1C, the photoresist layer 112 is removed and a conical-shape contact opening 114 is formed in the protective layer 110. The contact opening 114 serves mainly to expose the source/drain 108.
At present, it is common to use a composite multi-layered metal structure of molybdenum layer 108a/aluminum layer 108b/molydenum layer 108c as the source/drain 108. However, the molybdenum layer 108a is easily etched away by the reactive ions used in the etching process of the protective layer 110 to expose the underlying aluminum layer 108b. When the aluminum layer 108b is exposed to the surrounding air, a layer of thin aluminum oxide layer will form on the surface of the aluminum layer 108b. This aluminum oxide film will lead to a higher resistance with the conductive layer subsequently deposited into the contact opening 114. Ultimately, the quality of signal transmission between the conductive layer and the source/drain 108 is deteriorated.